Lieutenant (typically pronounced as "loo-tenant" or "lef-tenant" in British English) is a militaryrank, the equivalent of which is used by the service organizations of many civilizations. As a traditional naval rank, lieutenant is the grade between ensign and lieutenant commander. In infantry, lieutenant is the lowest officer grade, falling below captain. An infantry lieutenant commands a platoon, consisting of about thirty troops.

In the Starfleet, the rank of lieutenant was directly between ensign and commander. The FederationStarfleet uses additional grades in this transition, lieutenant junior grade, lieutenant and lieutenant commander. This is identical to the system used by many older Earth agencies. Starfleet lieutenants are expected to be effective small-section leaders and technical experts, developing their leadership skills in preparation for promotion to the commander tier.

In the Romulan Star Empire, there exists a lieutenant grade and also a lower grade, sublieutenant. In the old Earth British Royal Navy, the rank sublieutenant is equivalent to Lieutenant, Junior Grade.